Optical-resolution functional photoacoustic microscopy with fluence compansation

Functional optical-resolution photoacoustic microscopy (OR-PAM) can image blood oxygen saturation (sO ₂ ) in vivo with high resolution and high sensitivity. Especially, it can map the oxygen content in small vessels, which is of great importance in the neurovascular study and early cancer diagnosis. Conventional OR-PAM assumes that local fluence is identical for both wavelengths and can be calibrated. This method ignores the wavelength-dependent optical attenuation in superficial tissue. However, strong scattering in tissue may cause back-scattering and beam broadening and significantly change the optical deposition, leading to errors in sO ₂ measurement. To solve this problem, we develop OR-PAM with fluence compensation to correct the errors in sO ₂ imaging. We propose a linearized model to estimate the wavelengthdependent fluence attenuations and use three optical wavelengths to compensate for them in sO ₂ measurement. We validate the model using Monte Carlo simulations and phantoms. In functional brain imaging, we demonstrate that the fluence compensation method can effectively improve sO ₂ accuracy, especially in the small and deep vessels.